U.S. officials say a massive anti-terrorism exercise that culminated this week in two simulated terrorist attacks in the eastern United States, and one in Britain, put the government in a much better position to respond to any actual attack. And the officials say for the first time this exercise also included hints provided to intelligence and law enforcement agencies in recent weeks designed to test their ability to detect terrorist plans before they are put into effect.

To all appearances, the exercise began last weekend, as actors started to arrive at hospital emergency rooms in New Jersey complaining of flu-like symptoms. The symptoms were fake, of course, but by mid-day Monday, as U.S. Homeland Security officials began to see how many people were suddenly sick, they concluded that they may have been poisoned.

The final and most public stages of Operation "TOPOFF 3" had begun. But Bob Stephan, head of interagency incident management for the Department of Homeland Security, says the exercise had actually begun several weeks earlier with a new element never before tested.

"We began the exercise operational phase actually the first week in March with an intelligence and information buildup, which actually had built into it a couple of different preventable acts. If we had our act together with our state and local partners information-sharing-wise we should have been able to prevent a couple of attacks, which in fact we did during the course of the exercise," he said.

Mr. Stephan says intelligence and law enforcement agencies were provided with information about several possible terrorist attacks. The false information was delivered in small pieces, along with the regular information those agencies process every day. He reports that the authorities picked up on several suspicious individuals who had been planted in the northeastern United States, and arrested some of them, preventing their fake attacks from happening. But he says the authorities also ensured that two attacks would happen as planned, to test the response capabilities of local and federal agencies, and private organizations.

"We wanted to test the full range of our incident management processes and protocols that span prevention, intelligence and information sharing, and then the more traditional response and recovery," he said.

Those supposedly sick actors in New Jersey turned out not to have the flu, but rather to have been contaminated with pneumonic plague, a treatable but potentially fatal bacterial infection. Law enforcement authorities investigated and found a vehicle that was fitted with a device to spray the bacteria into the air. Officials identified the incident as a terrorist attack and launched a broader investigation, as well as a huge public health operation.

At about the same time, the exercise included another incident in nearby Connecticut. There, at a festival that attracted thousands of people, a fake car bomb went off, killing and injuring many people and releasing a phony chemical into the air that gave survivors simulated skin burns.

Officials say dozens of agencies were involved in responding to the fake attacks, and distributing information to the general public on what to do. There was also contact with the Canadian government, and there was a connection to an anti-terrorism exercise in Britain, as officials searched for a link between the phony incidents in the United States and a fake subway train bombing in London. Officials also evaluated how quickly they were able to get the details of the British exercise in order to issue a practice warning to U.S. transportation agencies. They say they did so within hours.

Mr. Stephan says the operation went well, and proved that efforts since the attacks of September 11, 2001 have paid off. And he says this week's exercise is already providing more lessons for the future. "We're acting even now on some things that we picked up during the exercise so that if an incident occurred tomorrow, we're going to be much better off," he said.

The full report on the exercise will not be ready for at least four months. But Homeland Security officials say they will begin sharing lessons learned with relief and law enforcement agencies throughout the country almost immediately.

And Mr. Stephan notes that his federal government team that responded to these fake crises will be involved in any future real incidents, wherever they happen. "The federal team that went through this exercise here, that's the same exact team that's going to take the country into the response piece (phase)if we do have a terrorism incident anywhere in the United States of America or its territories. This is it. So this team is now very well rehearsed, worked together in a very comprehensive manner," he said.

This was the largest such exercise in U.S. history. It took two years to plan, and involved 10,000 participants from 27 U.S. government agencies, including the military, and more than 200 local government agencies and private companies and organizations. The private groups were used as targets of the fake attacks, provided information on the impact of the attacks on the local people and economy, and provided volunteers, donations and blood for the relief efforts.

Officials will not give a specific figure, but they say the exercise involved several thousand fake deaths and thousands more injuries.

This time, the sick and dying were only acting. But officials are aware that someday there could well be a real attack. They say the more they learn about how to coordinate prevention and response efforts, the better job they will be able to do to minimize casualties if and when that happens.